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Jin Wu and Yi Wei

Abstract

An optical-electronic technique has been developed for simultaneous, remote measurements of the surface tension and wave attenuation over the water surface. The technique has been fully tested in a laboratory tank, and tried in the field. Sample results of measurements are presented to illustrate that the technique is an effective method for studying the surfactants on the air–sea interface.

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Jason E. Nachamkin and Yi Jin

Abstract

When consulting a forecast, users often ask some variant of the following questions: Will an event of interest occur? If so, when will it occur? How long will it last? How intense will it be? Standard verification measures often do not directly communicate the ability of a forecast to answer these questions. Instead, quantitative scores typically address them indirectly or in some combined form. A more direct performance measure grew from what started as a project for a high-school intern. The challenge was to evaluate aspects of forecast quality from a set of convection-allowing (1.67 km) precipitation forecasts over Florida. Although the output was highly detailed, evaluation became manageable by simply adding a series of static landmarks with range rings and radials. Using the “targets” as a guide, the student and the two authors successfully obtained quantitative estimates of model tendencies that had heretofore only been reported anecdotally. What follows is a description of the method as well as the results from the analysis. It is hoped that this work will stimulate a broader discussion about how to extract performance information from very complex forecasts and present that information in terms that humans can readily perceive.

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Qing Yun Zhao and Yi Jin

Major damage caused by hurricanes occurs over land during and after landfall. Accurate predictions of winds and precipitation in and around hurricanes at or near landfall are therefore of vital importance for hurricane preparation and damage mitigation, yet they continue to present a challenge for the hurricane research and numerical weather prediction (NWP) communities. This is, in part, due to rapid changes in hurricane intensity and structure during landfall associated with multiscale dynamical and physical interactions in the hurricane core regions and outer spiral rainbands, and also associated with sudden changes of surface conditions.

In this study, we demonstrate the capability of improving predictions of hurricane intensity and structures near landfall by assimilating high-resolution, three-dimensional observations from land-based radars in the landfall regions into a mesoscale NWP model. The landfall of Hurricane Isabel on the east coast of the United States in 2003 is the focus of this study. Observations of Doppler radial velocity and reflectivity from five Doppler radars in the landfall region were collected and assimilated into the Navy's Coupled Ocean-Atmosphere Mesoscale Prediction System in a variational data assimilation framework. Four cycles of hourly radar reflectivity data assimilation effectively correct the overprediction of hydrometeor fields by the model, and move the maximum reflectivity regions to the observed locations. Better hurricane structures, including increased maximum wind speed, a tighter inner core, and better organized outer rainbands, are obtained by the radar radial velocity assimilation. Much-improved forecasts of 24-h accumulated precipitation during and after hurricane landfall have also been achieved by the radar data assimilation. The positive results from this study indicate the potential for improving hurricane intensity and structure forecasts by assimilating radar observations into NWP models.

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Fengpeng Sun and Jin-Yi Yu

Abstract

This study examines the slow modulation of El Niño–Southern Oscillation (ENSO) intensity and its underlying mechanism. A 10–15-yr ENSO intensity modulation cycle is identified from historical and paleoclimate data by calculating the envelope function of boreal winter Niño-3.4 and Niño-3 sea surface temperature (SST) indices. Composite analyses reveal interesting spatial asymmetries between El Niño and La Niña events within the modulation cycle. In the enhanced intensity periods of the cycle, El Niño is located in the eastern tropical Pacific and La Niña in the central tropical Pacific. The asymmetry is reversed in the weakened intensity periods: El Niño centers in the central Pacific and La Niña in the eastern Pacific. El Niño and La Niña centered in the eastern Pacific are accompanied with basin-scale surface wind and thermocline anomalies, whereas those centered in the central Pacific are accompanied with local wind and thermocline anomalies. The El Niño–La Niña asymmetries provide a possible mechanism for ENSO to exert a nonzero residual effect that could lead to slow changes in the Pacific mean state. The mean state changes are characterized by an SST dipole pattern between the eastern and central tropical Pacific, which appears as one leading EOF mode of tropical Pacific decadal variability. The Pacific Walker circulation migrates zonally in association with this decadal mode and also changes the mean surface wind and thermocline patterns along the equator. Although the causality has not been established, it is speculated that the mean state changes in turn favor the alternative spatial patterns of El Niño and La Niña that manifest as the reversed ENSO asymmetries. Using these findings, an ENSO–Pacific climate interaction mechanism is hypothesized to explain the decadal ENSO intensity modulation cycle.

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Lei Wang and Jin-Yi Yu

Abstract

The tropospheric biennial oscillation (TBO) is conventionally considered to involve transitions between the Indian and Australian summer monsoons and the interactions between these two monsoons and the underlying Indo-Pacific Oceans. Here it is shown that, since the early 1990s, the TBO has evolved to mainly involve the transitions between the western North Pacific (WNP) and Australian monsoons. In this framework, the WNP monsoon replaces the Indian monsoon as the active Northern Hemisphere TBO monsoon center during recent decades. This change is found to be caused by stronger Pacific–Atlantic coupling and an increased influence of the tropical Atlantic Ocean on the Indian and WNP monsoons. The increased Atlantic Ocean influence damps the Pacific Ocean influence on the Indian summer monsoon (leading to a decrease in its variability) but amplifies the Pacific Ocean influence on the WNP summer monsoon (leading to an increase in its variability). These results suggest that the Pacific–Atlantic interactions have become more important to the TBO dynamics during recent decades.

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Jian Ma and Jin-Yi Yu

Abstract

This study analyzes representative concentration pathway 4.5 projections by 18 models from phase 5 of the Coupled Model Intercomparison Project to show that surface warming patterns in the equatorial Pacific during the twenty-first century (centennial warming) are influenced by the relative strengths of the Walker and Hadley circulations. The stronger the Hadley (Walker) circulation is, the greater the surface warming in the central Pacific (CP) [eastern Pacific (EP)]. The EP warming is associated with the Bjerknes feedback, while the CP warming is associated with the wind–evaporation–sea surface temperature feedback. This atmospheric circulation influence on the centennial warming is similar to that found for the EP and CP El Niño. This suggests a methodology to constrain the estimate of the projected surface warming patterns in the equatorial Pacific using recent El Niño activity. The constraint indicates that the “most likely” centennial warming patterns have a maximum in the EP and are 39% weaker than the warming projected by the 18-model mean. The most-likely projection also shows alternating stronger and weaker warming in the subtropical North Pacific, which is not predicted by the 18-model mean projection. Nevertheless, the two projections agree on the minimum warming in the southeastern subtropical Pacific.

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Hao Jin, Melinda S. Peng, Yi Jin, and James D. Doyle

Abstract

A series of experiments have been conducted using the Coupled Ocean–Atmosphere Mesoscale Prediction System–Tropical Cyclone (COAMPS-TC) to assess the impact of horizontal resolution on hurricane intensity prediction for 10 Atlantic storms during the 2005 and 2007 hurricane seasons. The results of this study from the Hurricane Katrina (2005) simulations indicate that the hurricane intensity and structure are very sensitive to the horizontal grid spacing (9 and 3 km) and underscore the need for cloud microphysics to capture the structure, especially for strong storms with small-diameter eyes and large pressure gradients. The high resolution simulates stronger vertical motions, a more distinct upper-level warm core, stronger upper-level outflow, and greater finescale structure associated with deep convection, including spiral rainbands and the secondary circulation. A vortex Rossby wave (VRW) spectrum analysis is performed on the simulated 10-m winds and the NOAA/Hurricane Research Division (HRD) Real-Time Hurricane Wind Analysis System (H*Wind) to evaluate the impact of horizontal resolution. The degree to which the VRWs are adequately resolved near the TC inner core is addressed and the associated resolvable wave energy is explored at different grid resolutions. The fine resolution is necessary to resolve higher-wavenumber modes of VRWs to preserve more wave energy and, hence, to attain a more detailed eyewall structure. The wind–pressure relationship from the high-resolution simulations is in better agreement with the observations than are the coarse-resolution simulations for the strong storms. Two case studies are analyzed and overall the statistical analyses indicate that high resolution is beneficial for TC intensity and structure forecasts, while it has little impact on track forecasts.

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John D. Farrara and Jin-Yi Yu

Abstract

The interannual variability in the southwest U.S. monsoon and its relationship to sea surface temperature (SST) anomalies is investigated via experiments conducted with the University of California, Los Angeles, atmospheric general circulation model (AGCM). When the model is run without interannual variations in SSTs at the lower boundary, the simulation of the climatological mean monsoon is quite similar to the observed. In addition, the interannual precipitation variance and wet minus dry monsoon composite differences in the precipitation and monsoon circulation are largely realistic.

When interannual variations in SSTs are introduced, the simulated interannual precipitation variance over the southwest U.S. monsoon region does not increase. Nor do SSTs seem to be important in selecting for wet or dry monsoons in this simulation, as there is little correspondence between observed wet and dry monsoon years and simulated wet and dry years. These results were confirmed through a 20-member ensemble of shorter seasonal simulations forced by an SST anomaly field corresponding to that observed for a wet minus dry southwest U.S. monsoon composite.

When the AGCM is coupled to a mixed-layer ocean model, the pattern of SST anomalies generated in association with wet and dry monsoons is remarkably similar to that observed: there is a large area of positive SST anomalies in the subtropical eastern Pacific Ocean and weaker negative anomalies in the midlatitude North Pacific and Gulf of Mexico. It is demonstrated that the SST anomalies in the Pacific Ocean are forced by anomalies in the net surface solar radiative flux from the atmosphere associated with variations in planetary boundary layer stratus clouds; these variations are enhanced by a positive feedback between SST and stratus cloud variations. The anomalies in the Gulf of Mexico are associated with anomalous latent heat fluxes there. It is concluded that internal atmospheric variations are capable of 1) producing interannual variations in the southwest U.S. monsoon that are comparable to those observed, and 2) thermodynamically forcing the SST anomalies in the adjacent Pacific Ocean and Gulf of Mexico that are observed to accompany these variations. The implications of these results for seasonal forecasting are rather pessimistic since variations associated with internal atmospheric processes cannot be predicted on seasonal timescales.

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Jin-Yi Yu and Carlos R. Mechoso

Abstract

The hypothesis that Peruvian stratocumulus play an important role on both the annual mean and annual variations of sea surface temperature (SST) in the eastern equatorial Pacific is examined. The problem is addressed by performing sensitivity experiments using the University of California, Los Angeles, coupled atmosphere–ocean GCM with different idealized temporal variations of stratocumulus in a region along the coast of Peru.

The results obtained are consistent with the notion that Peruvian stratocumulus are a key component of the interhemispherically asymmetric features that characterize the annual mean climate of the eastern equatorial Pacific, including the cold SSTs off Peru and the absence of a southern ITCZ. The principal new finding of this study is that the annual variations (i.e., deviations from the annual mean) of Peruvian stratocumulus are linked to the differences between the amplitude, duration, and westward propagation of the warm and cold phases of the equatorial cold tongue. In the model’s context, only if the prescribed annual variations of Peruvian stratocumulus have the same phase as the observed variations are those differences successfully captured.

The impact of Peruvian stratocumulus on equatorial SST involves “dynamical” and “thermal” effects. The former develop through an enhancement of the northerly component of the surface wind from the Peruvian coast to the equator. The thermal effects develop through the special relationships between SST and surface evaporation over the equatorial cold tongue, which contributes to extend the cold phase until the end of the year. A successful portrayal of this behavior requires a realistic simulation of the annual variations of surface wind over the equatorial cold tongue.

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Kewei Lyu, Jin-Yi Yu, and Houk Paek

Abstract

The Atlantic multidecadal oscillation (AMO) has been shown to be capable of exerting significant influences on the Pacific climate. In this study, the authors analyze reanalysis datasets and conduct forced and coupled experiments with an atmospheric general circulation model (AGCM) to explain why the winter North Pacific subtropical high strengthens and expands northwestward during the positive phase of the AMO. The results show that the tropical Atlantic warming associated with the positive AMO phase leads to a westward displacement of the Pacific Walker circulation and a cooling of the tropical Pacific Ocean, thereby inducing anomalous descending motion over the central tropical Pacific. The descending motion then excites a stationary Rossby wave pattern that extends northward to produce a nearly barotropic anticyclone over the North Pacific. A diagnosis based on the quasigeostrophic vertical velocity equation reveals that the stationary wave pattern also results in enhanced subsidence over the northeastern Pacific via the anomalous advections of vorticity and temperature. The anomalous barotropic anticyclone and the enhanced subsidence are the two mechanisms that increase the sea level pressure over the North Pacific. The latter mechanism occurs to the southeast of the former one and thus is more influential in the subtropical high region. Both mechanisms can be produced in forced and coupled AGCMs but are displaced northward as a result of stationary wave patterns that differ from those observed. This explains why the model-simulated North Pacific sea level pressure responses to the AMO tend to be biased northward.

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